"ringing" sound, on certain notes (PR w new OT)

Discussion in 'Shock Brother's DIY Amps' started by LudwigvonBirk, Jul 11, 2019.

  1. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I recently replaced the output transformer on a Princeton Reverb clone (no other changes at all). After the OT swap, I now hear a light "ringing" type of sound on certain notes that wasn't happening with the prior OT. Almost like "reverb spring shimmer". I don't think you would hear this when playing with a group, but you would if you recorded a solo with the amp.

    Tube swaps (one at a time, all positions incl rectifier) - no difference.
    Reverb off- no difference.
    Trem off - no difference.
    Volume up or down, no difference.
    Chassis in or out of the cab - no difference.

    The amp does sound *very good now* and I'd like to keep the new OT in there.

    Any thoughts on how I might address this?

    (Increase the NFB resistor value a bit maybe?)
     
  2. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Seems unlikely, but I recall that OT squeal may not be a squeal, it may be all sorts of odd sounds. Have you reversed the OT leads?
     
  3. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Have not, could try it.

    (I have heard backwards-lead squeal before and it was WAY more extreme than this fwiw)
     
  4. tubeswell

    tubeswell Friend of Leo's

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    If the new OT has more interleaving for the windings, the inter-winding capacitance can produce a frequency phase shift that interacts with the NFB loop and produces the symptoms you describe. Is your amp's FB loop signal insertion point as-per the standard PR schematic?

    You could add 250pF from each output tube grid to each cathode; in Merlin's parlance this is 'slugging the dominant pole' of the FB loop. It won't affect the audible frequency response of the amp, but will help eliminate the ringing. Fender put these on their SF amps in the early 70s because of this.
     
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  5. schmee

    schmee Poster Extraordinaire

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    ^^Try this. Some don't squeal but make other issues. No idea why that is.
     
  6. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Yes- PR standard method, matches schematic.

    Interesting!
    I will try this^ after swapping the OT leads, just to rule that one out.
     
  7. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I believe you, but I haven't (yet) found a schematic and/or wiring diagram that shows this. Will keep looking...
     
  8. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Did any of the wiring around the tube sockets get moved during the OT swap? Perhaps a slight lead dress adjustment will help if swapping the OT leads does not work.
     
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  9. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Good idea. If an OT lead switch doesn't help, ringing on certain notes suggests microphonics somewhere. As Dan says, wires, and solder joints, and almost anything that can wiggle can develop harmonic vibration in sympathy with the speaker. I see you tried tube swaps, one at a time. But did the OT installation have some side effect on a wire, solder joint, tube socket, or pin? It doesn't *have* to be a tube, though I had one 'reveal' it was microphonic after some mods.
     
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  10. tubeswell

    tubeswell Friend of Leo's

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    Here you go
     

    Attached Files:

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  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Thanks!
     
  12. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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  13. schmee

    schmee Poster Extraordinaire

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    There should be no need for adding those caps, find out what is wrong instead.
     
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  14. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I can now confirm the above- I played out last night with a 6-piece + singer, nobody could hear "my problem", including me while sitting right next to amp. The drummer is an EE and audio-circuit designer and I asked during a break if he could hear it...he said nope.
     
  15. tubeswell

    tubeswell Friend of Leo's

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    2nF is overkill and dulls the high end somewhat, but 250pF* will work without affecting audible frequencies.

    *use high voltage caps (500V or higher)

    If the problem is related to NFB HF phase shift because of your new OT, this is one ‘fix’. (If the problem is caused by some other source of HF/RF oscillation, then you could try sizeable (~47k) grid stoppers on the output tube grids - but you didn’t have this problem with the previous OT, right?)
     
    Last edited: Jul 12, 2019
  16. tubeswell

    tubeswell Friend of Leo's

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    The problem started when a new OT was installed. Positive feedback has already been ruled out by the thread poster. There can only be a small number of possible reasons for the symptoms the poster describes. I suspect one of them (HF phase shift resulting from interwinding capacitance reacting with the global NFB loop). What else do you think the problem could be?
     
  17. schmee

    schmee Poster Extraordinaire

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    Just sayin': It worked before. BF amps work fine without those caps. (which most people remove from SF amps). The caps are a band aid.
     
  18. THX1123

    THX1123 TDPRI Member

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    This might sound odd, and I am probably way off here, but....maybe it isn't the amp. Could it be that you are hearing the strings above the nut ringing now that the amp sounds much better? I have to dampen my 6 on a side guitars up there, it is just an artifact of how and where I strike the things. The sound the OP describes reminds me of that sound, it is like a very high pitched ringing that decays quickly, and it does disappear into a live band.
     
  19. Wally

    Wally Telefied Ad Free Member

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    Tubeswell, thanks for the insight as to the purpose of those caps. However, I see 2000pfd and 1200pfd caps used there as opposed to 250 pad. ???? Am I seeing things incorrectly?
     
  20. tubeswell

    tubeswell Friend of Leo's

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    Yes you ain’t seeing things. 2nF or even 1n2 is too high for NFB slugging. If the amp suffers from ultrasonic ringing (due to the OT), then you need to intervene somehow. Some amps use a feedback cap between the LTP plates, some use plate bypass caps on the LTP. You don’t have a LTP in a PR, and so another way of addressing the problem is grid HF bypass caps. 250pF won’t affect the audible frequency response.
     
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